Various systems have been proposed heretofore for transmitting data and/or control signals as well as electrical power over one or more electrical conductors interconnecting the surface equipment and sub-surface apparatus such as perforating guns, various downhole measuring devices, or controls for subsea well heads. Those skilled in the art will appreciate, however, that when the sub-surface apparatus is located in a pipe string it is difficult to provide a continuous trouble-free electrical communication path between the sub-surface apparatus and surface equipment. The simplest technique is, of course, to dependently couple the sub-surface apparatus to an electrical cable and then temporarily remove the apparatus and its supporting cable from the pipe string each time that a pipe joint is to be removed or added to the pipe string. This straight-forward technique is particularly useful for stationing a measuring instrument in a tubing string in a completed well bore and thereafter obtaining measurements as desired. Nevertheless, when this technique is used to make various measurements during the course of a typical drilling operation, there will be a significant increase in the amount of time required to carry out even the simplest downhole measurement. An example of this time-consuming technique is seen in U.S. Pat. No. 3,789,936.
Accordingly, to minimize the number of times that a measuring device has to be removed from the drill string during a drilling operation, as shown, for example, in U.S. Pat. No. 3,825,078, it has been proposed to support measuring instruments by an electrical cable that has an upper portion of considerable excess length that is arranged in one or more doubled loops in the upper portion of the drill string. A similar arrangement is seen in U.S. Pat. No. 4,416,494 where the extra portion of the cable is instead coiled within a special container disposed in the drill string. In either case, by arranging an electrical connector on the upper end of the cable, the upper end portion of the cable can be quickly disconnected from the surface equipment. In this manner, the upper end portion of the cable can be readily passed through a pipe joint that is either being removed from or added to the upper end of the drill string. The cable is then reconnected to the surface equipment and the drilling operation is again resumed. Additional sections of cable are periodically added to the upper portion of the cable to increase the overall length of the cable as the drilling operation continues to deepen the borehole. Despite the time-saving features offered by these complicated handling techniques, there is always a chance that the extra cable portion will become twisted or entangled within the drill pipe. Moreover, since additional cable sections are coupled to the main cable, there will be an increasing number of electrical connectors in the drill string which are subjected to the adverse effects of the drilling mud passing through the drill string.
To avoid the handling problems presented by a cable that is loosely disposed within a pipe string, it has also been proposed to provide an electrical conductor that is secured to or mounted in the wall of each pipe joint. For example, as shown in U.S. Pat. No. 2,748,358, a short length of electrical cable is arranged in each pipe joint and supported therein by way of an electrical connector that is coaxially mounted in an upstanding position just inside of the female or so-called "box end" of the pipe joint. The lower end of the cable is unrestrained and is allowed to hang just below the so-called "pin end" of the pipe joint so that the electrical connectors can be mated and the pipe string assembled or disassembled without unduly disturbing the cable lengths or their mated connectors. Similar arrangements are disclosed in U.S. Pat. No. 3,184,698 and U.S. Pat. No. 3,253,245. Another proposed arrangement shown in U.S. Pat. No. 4,399,877 utilizes a so-called "side-entry sub" which is coupled in the pipe string and has an opening in one side wall through which an electrical cable can be passed.
In the systems shown in the several aforementioned patents, their respective electrical connectors must be manually connected as pipe string is moved into the well bore. To avoid wasting the time required for manually connecting a large number of connectors, as shown in U.S. Pat. No. 4,095,865 and U.S. Pat. No. 4,220,381, it has been proposed to also provide mating contacts in the ends of each of the pipe joints which will be automatically connected as the pipe joints are coupled together. With either of these design arrangements, it will, of course, be appreciated that there is always a substantial risk that one or more of the connectors required to interconnect so many short cables will be adversely affected by the well bore fluids.
In view of the many problems typically associated with electrical connectors, it has been proposed to instead provide inductive couplings on the opposite ends of the pipe joints for interconnecting the cables in each pipe joint. U.S. Pat. No. 2,379,800, for example, shows a typical set of induction coils that are respectively wound on annular soft-iron cores mounted in opposing recesses on the ends of each joint and cooperatively arranged so that whenever the pipe joints are tandemly coupled together each pair of coils will provide a transformer coupling between the cables in those pipe joints. U.S. Pat. No. 3,090,031, for example, attempts to overcome the inherently-high losses of conventional transformer couplings within typical oilfield piping by providing an encapsulated transistorized amplifier and power source at each associated pair of inductive windings.
To avoid the various problems discussed above, it has also been proposed to mount one or more measuring devices in the lower end of the pipe string and inductively couple these devices to an electrical cable that is lowered through the pipe string to the downhole measuring devices. For instance, as seen in FIGS. 2 and 7 of U.S. Pat. No. 2,370,818, a measuring device which is mounted in a drill collar coupled to the lower end of the drill string is provided with an output coil that is coaxially disposed in an annular recess around the inner wall of the drill collar. The output signals are transmitted to the surface by way of an electrical cable having a matching coupling coil on its lower end that is wound around a central ferromagnetic core member arranged to be complementally fitted into the output coil on the measuring device.
U.S. Pat. No. 3,209,323 discloses a similar measuring system having a measuring device which is adapted to be mounted on the lower end of a drill string and cooperatively arranged for transmitting signals to and from the surface by way of a matched pair of induction coils which are respectively arranged within an upstanding fishing neck that is coaxially disposed in the drill collar on top of the measuring device and a complementally-sized overshot that is dependently suspended from a typical electrical cable. Although this particular arrangement eliminates many of the problems discussed above, it will be recognized that since these induction coils are surrounded by thick-walled drill pipe, a significant amount of electrical energy that could otherwise be transferred through these coils will instead be dissipated into the electrically conductive pipe. Thus, it will be appreciated by those skilled in the art that with this prior-art arrangement, the unavoidable loss of electrical energy will be so great that the system simply cannot transmit signals to and from the surface unless these coils are closely fitted together. This need for a close fit between these induction coils will, therefore, make it difficult to lower the overshot through the drill string with any assurance that it can be reliably positioned around the fishing neck. Moreover, in those situations where well bore debris has accumulated around the upstanding fishing neck on the measuring device before the overshot is lowered into the drill string, the debris could make it difficult or impossible to properly position the overshot on the fishing neck.
The various problems associated with the several data-transmission systems discussed in the aforementioned patents are similar in many respects to the problems associated with coupling a surface power source to a typical oilfield perforating device. Accordingly, as seen in U.S. Pat. No. 4,544,035, a perforating gun that is adapted to be run into a well on the lower end of a tubing string is provided with an inductive coupling arrangement that is generally similar to the coupling arrangement disclosed in the above-mentioned U.S. Pat. No. 3,209,323.
Despite the proliferation of patents involving various systems of this nature it is readily apparent to those skilled in the art that none of the systems discussed above for transmitting signals and/or power between the surface and downhole devices in a pipe string have been commercially successful. Instead it has been necessary heretofore either to use a continuous electrical cable that is directly connected to the downhole equipment for transmitting data and power or to utilize a so-called measuring-while-drilling or "MWD" tool with a self-contained power supply which is cooperatively arranged for sending data to the surface by transmitting acoustic signals through the drill string fluid.